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November 2006

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Subject:
From:
"Whittaker, Dewey (EHCOE)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Whittaker, Dewey (EHCOE)
Date:
Tue, 7 Nov 2006 15:11:34 -0700
Content-Type:
text/plain
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text/plain (131 lines)
You and Werner make excellent points. For those same reasons, I have
forbidden the use of these parts for many years. If they must be used, I
require special mounting instructions and a disclaimer on any long-term
reliability commitments.
Dewey

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of JaMi Smith
Sent: Tuesday, November 07, 2006 2:31 PM
To: [log in to unmask]
Subject: Re: [TN] Gold Embrittlement

Werner,

Looking strictly at the top pictures, it can be seen that the leads are
bent
slightly downwards, and only contact the PCB traces at the ends.
Additionally, the ground leads are pathetically short, and do not even
appear to be in full contact with the traces across their width. In this
specific instance, trying to argue thermal mismatch between the base
material of the leads and the copper of the PCB trace doesn't even
appear to
be applicable, due to the minimal contact.

Respecting any thermal mismatch of your Alloy 42 and Cu, we have been
building RF Amplifiers and other curcuits for years, even in spacecraft,
without having the problems you espouse, especially to the extent that
you
imply, and once again I would argue that the ground leads shown in the
pictures are way way too short, and I might add that cutting them at an
angle does absolutely nothing for them except make for much less thermal
transfer and electrical transfer.

Having the lead flat on the copper trace, is not only an electrical
issue
(current transfer), but also an RF issue (impedance mismatch), and while
I
can accept that the leads should not be excessively long, neither should
they be excessively short.

Now as for the gold embrittlement itself, I have a few questions.

Does the length of time that the joint is held at solder flow/reflow
temperature allow for more of the gold to go into solution with the
solder?

Does the amount of solder that is added to the solder joint allow for
more
of the gold to go into solution with the solder?

What does gold embrittlement look like, or does it even happen, and if
so to
what extent, with the new solders that are being used as we go 'lead
free'?

Are there any specific 'lead free' solders that are better in avoiding
gold
embrittlement that the others, and why?

JaMi


----- Original Message -----
From: "Werner Engelmaier" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, November 07, 2006 12:44 PM
Subject: Re: [TN] Gold Embrittlement


> Hi 'JaMi,'
> While I agree with you about the appearance of these solder joints, I
cannot
> agree with you regarding what would happen in the absence of Au.
> If you make the leads and their attachment area larger [which from
electrical
> and thermal conductivity considerations is advantageous], you run into
> another problem.
> For this type of devices, the lead material is typically Alloy 42, not
Cu.
> Thus, large soldered areas will suffer from local expansion mismatches
in
> combination with the global expansion mismatches.
> In any case, the excessive amount of Au does not help.
>
> Werner
>
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